Re: [Qemu-devel] [V6 0/4] AMD IOMMU

[Jan Kiszka]

On 2016-03-01 15:12, Jan Kiszka wrote:
> On 2016-03-01 14:55, Michael S. Tsirkin wrote:
>> On Tue, Mar 01, 2016 at 02:48:19PM +0100, Jan Kiszka wrote:
>>> On 2016-03-01 14:07, Michael S. Tsirkin wrote:
>>>> On Sun, Feb 21, 2016 at 09:10:56PM +0300, David Kiarie wrote:
>>>>> Hello there,
>>>>>
>>>>> Repost, AMD IOMMU patches version 6.
>>>>>
>>>>> Changes since version 5
>>>>>  -Fixed macro formating issues
>>>>>  -changed occurences of IO MMU to IOMMU for consistency
>>>>>  -Fixed capability registers duplication
>>>>>  -Rebased to current master
>>>>>
>>>>> David Kiarie (4):
>>>>>   hw/i386: Introduce AMD IOMMU
>>>>>   hw/core: Add AMD IOMMU to machine properties
>>>>>   hw/i386: ACPI table for AMD IOMMU
>>>>>   hw/pci-host: Emulate AMD IOMMU
>>>>
>>>> I went over AMD IOMMU spec.
>>>> I'm concerned that it appears that there's no chance for it to
>>>> work correctly if host caches invalid PTE entries.
>>>>
>>>> The spec vaguely discusses write-protecting such PTEs but
>>>> that would be very complex if it can be made to work at all.
>>>>
>>>> This means that this can't work with e.g. VFIO.
>>>> It can only work with emulated devices.
>>>
>>> You mean it can't work if we program a real IOMMU (for VFIO) with
>>> translated data from the emulated one but cannot track any updates of
>>> the related page tables because the guest is not required to issue
>>> traceable flush requests? Hmm, too bad.
>>>
>>>>
>>>> OTOH VTD can easily support PTE shadowing by setting a flag.
>>>
>>> Do you mean RWBF=1 in the CAP register? Given that "Newer hardware
>>> implementations are expected to NOT require explicit software flushing
>>> of write buffers and report RWBF=0 in the Capability register", we may
>>> eventually run into guests that no longer check that flag if we expose
>>> something that looks like a "newer" implementation.
>>
>> Hopefully not, if that happens we'll have to do a PV IOMMU :)
> 
> Please not.
> 
>>
>>> However, this flag is not set right now in our VT-d model.
>>>>
>>>> I'd like us to find some way to avoid possibility
>>>> of user error creating a configuration mixing e.g.
>>>> vfio with the amd iommu.
>>>>
>>>> I'm not sure how to do this.
>>>>
>>>> Any idea?
>>>
>>> There is likely no way around write-protecting the IOMMU page tables (in
>>> KVM mode) once we evaluated and cached them somewhere.
>>
>> Well for one, it's possible to use vt-d and not amd iommu.
> 
> That would lead to nice combos of AMD CPUs with VT-d IOMMU. While it may
> be possible, I wouldn't rely on guests having tested that combination
> very well.

To make the concern more concrete: I'm playing with code that will reuse
the MMU page tables for the IOMMU - the AMD architecture is designed for
that optimization (in contrast to Intel's). So, if the guest is not
foreseeing that artificial combo above (ours will definitely not)
because it is designed around the reuse, it will at least fail to run.

Jan

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